N30 and the effect of explorative finger movements: a model of the contribution of the motor cortex to early somatosensory potentials

Objectives: The source of the N30 potential in the median nerve somatosenso ry evoked potentials (SEP) has been previously attributed to a pre-central origin (motor cortex or the supplementary motor area, SMA) or a post-centra l located generator (somatosensory cortex). This attribution was made from results of lesion studies, the behavior of the potential under pathological conditions, and dipole source localization within spherical volume conduct or models. Methods: The present study applied dipole source localization and current d ensity reconstruction within individual realistically shaped head models to median nerve SEPs obtained during explorative finger movements. Results: The SEPs associated with movement of the stimulated hand showed a minor reduction of the N20 amplitude and a markedly reduced amplitude for t he frontal N30 and parietal P27, exhibiting a residual frontal negativity a round 25 ms, The brain-stem P14 remained unchanged. Mapping of the differen t SEPs (movement of the non-stimulated hand minus movement of the stimulate d hand) showed a bipolar field pattern with a maximum around 30 ms post-sti mulus. In eight out of ten normal subjects, both the N30 and the gN30 (subt raction data) sources resided within the pre-central gyrus, more medially t han the post-centrally located N20. Two subjects, in contrast, showed rathe r post-centrally localized sources in this time range. A model of the corti cal SEP sources is introduced, explaining the data with respect to previous ly described findings of dipole localization, and from lesion studies and t he alterations seen in motor diseases. Conclusions: The results provide evidence for a pre-central N30 generator, predominantly tangentially oriented, located within the motor cortex, while no sources were detected elsewhere. It is suggested that the mechanisms un derlying the 'gating' effect during explorative finger movements in the 30 ms time range predominantly arise in the motor cortex. (C) 1999 Elsevier Sc ience Ireland Ltd. All rights reserved.